02 November 2011

To not boldly go anywhere.

I wrote this some time back after reading Tom Murphy's blog. I'll publish this as is because it is still worth saying even though it is really more shallow than I wanted.

I was reading Tom Murphy's blog, Do The Math, about the future of space flight. In a nutshell, it has no future. These are hard words for many people raised with the dream-become-expectation that people (usually American) will venture into space and colonise other worlds.

I sympathise. When I first realised that civilisation was in dire straits because: no we had not developed fusion; no we had not developed safe/cheap nuclear; no we had not developed a cheap replacement for petroleum and no we had not developed a better means of getting into space -- well I was not happy. In fact I found it deeply depressing, because even rudimentary calculations show that our current civilisation cannot continue as is. A relevant motto comes to mind: per ardua ad astra often translated as, "A rough road leads to the stars". You don't get there by dreaming and you shouldn't let your expectations outrun your dreams.

There is a lot of energy being thrown about in the universe. Nature likes to show off on the grand scale. Sadly we can only intercept a tiny fraction and then we have to cajole it, at cost, into another form that we can use. We can look longingly at the lakes of hydrocarbons on Titan or the vast energy output of a solar prominence but we can't tap it. Just as the people of the neolithic could not tap the hydro-power of a waterfall. But they could at least walk up to it, perhaps even dream of a water wheel. We cannot just walk up to these things.

Is there any future for Space Flight/Travel/Exploration? I don't know. The trouble is that it takes a lot of complex interacting industries to create and to support a spacecraft. It is Rocket Science after all. But when the global energy pie starts shrinking then the web of requirements needed to make a spaceship starts fraying and breaking and the reliability of a launch will suddenly drop to zero. Long before we get to that stage I think people in power, ie politicians, will start pointing out that there are other demands for the energy/materials that the launch requires.

Perhaps we could, after a rough patch, develop fusion and rebuild and head off to space again. Is that possible? At this point it is worth mentioning a simple equation from basic mechanics, it is the Rocket Equation:

basically you can see that you need lots of fuel but more importantly a high exhaust velocity for your rocket. In fact twice the exhaust velocity will give you twice the final velocity. You can play around with the values and get all sorts of interesting results. For example, if you have a rocket that can throw empty bits of rocket away you can reduce your final rocket mass value and therefore increase the velocity. This is the idea behind multi-stage rockets. Or instead you could have a high exhaust velocity. Well that is hard to do unless you have something like an ion drive which has high exhaust velocity but not many particles and hence not much thrust ... which means you have to have a small craft and run the engine for a long time. No-one said the final velocity was reached in short time. Good for small unmanned explorer craft. Chemical energy in conventional rockets can give us a lot of thrust but with a relatively low exhaust velocity. Ion rockets give high exhaust velocity but low thrust. It would be nice to have something else. Many authors have played with the idea of nuclear fusion as a method of generating power and propulsion on a spacecraft. Looking at some simple figures shows us why (the last column relates to the final velocity)

Exhaust Velocity (m/s) Rocket Mass (kg) Rocket Fuel (kg) Days to Travel 100Mkm Exhaust Temp K
1000 80000 50000 2384 80200
5000 80000 50000 477 2006450
10000 80000 50000 238 8020000
20000 80000 50000 119 32000000
30000 80000 50000 79 72000000

Notice that the exhaust temperature* rapidly goes into the millions of degrees. Here I am assuming a deuterium exhaust. A velocity over 10,000 m/s seems 'unrealistic' even considering we are in pure speculation mode.

Caveat: I have made no allowance for the huge gravity well of the solar system. Even with such fusion motors you would have to play it smart and not use wasteful 'direct' paths to destinations.

I don't see it happening, well not the way our society is at the moment. If we had cheap fusion ... well that is dreaming again. Waiting for the aliens to come and rescue us, or the secret government free-energy devices that have been suppressed. Yeah right. Might happen, I wouldn't like to bet my civilisation on it though.

So will humans ever venture into space again in a serious, sustained manner? I want to think so. But the only way I could see it happening is that humans build a civilisation that was frugal, innately curious and interested in knowledge and was very, very smart. I don't see that happening with Homo Sapiens. Fast forward 100,000 years. The CO2 in the atmosphere gets picked up by the carbon cycle. The natural climate re-asserts itself with the beginning of a new glacial period. Human populations contract, get pressured ... and pressured. A new interglacial starts with a new species of human. This is starting to sound a bit like Last and First Men by Olaf Stapledon.

Well that is all very depressing. Is there an alternative? The more we wish to go physically to a place the more it will cost. Manned spaceflight to Mars is horrendously expensive and the rewards are debatable; Unmanned spaceflight to Mars is somewhat expensive but the rewards are great; Remote observation of Mars is routinely done, inexpensive and often has surprising discoveries. Umanned exploration and remote observation may keep humanity's dream alive while some alternative is developed. Failing that, remote observation could fade into a deep tribal knowledge of the heavens that may survive to better times. Amazing successors to Stonehenge? Never forget that humans have studied the heavens for many thousands of years.

So where to now? I don't have any answers. Do you?

* Exhaust temperature is calculated by back of the envelope calculation using the V_rms in the Maxwell Speed Distribution equation.

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